Guayule rubber can be recovered from the guayule shrub Parthenium Argentatum which grows in semiarid regions. There has been a renewed interest in the commercialization of guayule rubber to supplement natural rubber obtained from the rubber tree Hevea brasiliensis. Both guayule and Hevea are considered "natural rubber."
Unlike Hevea, guayule rubber does not contain natural antioxidants and must be stabilized to prevent rapid degradation upon contact with air (Guayule, An Alternate Source of Natural Rubber; National Academy of Science; Washington D.C. 1977; page 25). An antioxidant to stabilize guayule rubber is usually added during the solution phase purification of the crude guayule rubber (ibid, p. 36). Since guayule and Hevea rubbers have chemical and physical properties that are virtually identical (ibid, p. 3), it is reasonable to expect that conventional antioxidants for rubber (specifically, cis-1,4-polyisoprene) would be adequate for the stabilization of guayule rubber. However, Keller, Winkler and Stevens (Paper No. 55, "Degradative Effects of Guayule Resin on Natural Rubber," presented at the 117th Meeting of the Rubber Division, ACS; Las Vegas, Nev., May 1980, determined that linoleic acid, which is a component of guayule resin (which in turn is a potential contaminant of guayule rubber) has an adverse effect on the stability of guayule rubber. Therefore, from a practical standpoint, a stabilizer additive(s) for guayule rubber preferably will function to prevent the deleterious effects of heat and oxygen (air) on the rubber itself (cis-1,4-structure) as well as the possible adverse effects of certain components of guayule resin.
There are many chemical compounds which have been shown to be effective antioxidants for elastomers. For example, Dunn in his article, "Review of Unsolved Problems in the Protection of Rubber Against Oxidative Degradation," Rubber Chemistry and Technology, 47 960 (1974); pointed out that there were over three thousand patents issued in a ten year period on the protection of polymers against aging. Dunn also pointed out that because of "antagonism," an antioxidant which proves highly effective in one rubber can be totally ineffective in another. Howard [Rubber Chemistry and Technology 47 976 (1974)] discussed antioxidant synergism.
The following references disclose the use of specific compounds as stabilizers for guayule rubber.
Gonzales (Paper No. 2, 116th Meeting of the Rubber Division, ACS, Cleveland, Ohio, Oct. 23-26, 1979) reported the results of stabilization of guayule rubber with phenolic compounds (thio-, methylene coupled as well as alkylated) and N,N'-diphenyl-p-phenylenediamine. The synergistic results of our invention is not suggested by Gonzales.
Butyl Zimate.RTM. which is a product of R. T. Vanderbilt Co., Inc. and chemically identified as the zinc salt of di-n-butyldithiocarbamic acid has been advertized as an excellent stabilizer for guayule rubber (Chemical & Engineering News, May 11, 1981).
The C & EN May 11, 1981 advertisement also disclosed that Butyl Zimate.RTM. alone or with Agerite.RTM. Geltrol.RTM. is the optimum stabilizer for Kraton.RTM. 1107 - based hot melts. Agerite.RTM. Geltrol.RTM. is also a product of R. T. Vanderbilt and is identified chemically (Rubber World; Materials and Compounding Ingredients for Rubber, 1968 Edition, page 97) as a modified high molecular weight hindered phenol. Hindered phenols are not included in the synergistic combination of the present invention.
A somewhat more specific identification of Agerite.RTM. and Geltrol.RTM. is an alkylated-arylated bisphenolic phosphite (The Vanderbilt Handbook, page 388).
Baldemar Motomochi B, Pasquale G. Galioto and Hans R. Strop disclosed in their publication entitled, "Mechanical Drying of Guayule and Hevea: A Pilot Plant Study) a paper presented at 3rd International Guayule Conference, Pasadena, Calif., Apr. 30, 1980). the use of 50 percent Butyl Zimate/50 percent AgeRite Superlite.RTM. at a 1 percent loading in guayule rubber as a stabilizer system. AgeRite Superlite.RTM. was identified as a polybutylated Bisphenol A which is a phenolic compound and no mention of synergism was made.
"BHT" which is butylated hydroxy toluene was disclosed and the antioxidant used to stabilize guayule rubber in the Saltillo, Mexico pilot plant (Paper No. 19, "Vulcanization of Guayule Rubber;" L. F. Ramos and B. Motomochi presented at the 116th meeting of the Rubber Division, ACS, Oct. 23-36, 1978 in Cleveland, Ohio.
U.S. Pat. Nos. 1,753,184 and 1,753,185 relate to the use of dimethyl-para-phenylenediamine as a stabilizing agent for guayule rubber. However, these patents fail to suggest the utilization of a second stabilizing compound or any synergistic result obtained thereby.
No reference relating to the use of a positive synergistic combination of para-phenylenediamines with organic phosphites for the stabilization of elastomers such as guayule rubber has been found.